It has been common to manufacture a flexible printed wiring board that comprises a board made of, for example, polyimide film with conductor wiring formed on one side or both sides thereof by the so-called subtractive method or semi-additive method
In the subtractive method, it has been common to use a Cu-clad board (sputter-plated board, casting board, laminated board, etc.) comprising an insulating film such as polyimide film having Cu layer formed on one side or both sides thereof, with a circuit formed from the Cu layer by removing the unnecessary portion of the layer by selective etching, and forming an etching resist film used in etching by lithography in a pattern corresponding to the desired conductor wiring.
However, lithography requires it to coat the Cu layer with the resist, expose the resist to irradiation in a predetermined pattern and remove the unnecessary portions in a developing process so as to form the etching resist mask, while it is also necessary to remove the resist mask after the etching process.
Thus the subtractive method involves a large number of processes including the processes related to etching, resulting in low productivity of manufacturing the printed wiring board. In addition, use of the expensive Cu-clad board and the resist results in a significant increase in the manufacturing cost.
In the semi-additive method, a board having an electrically conductive layer formed over the entire surface thereof by sputtering or the like is provided with conductor wiring by forming thin metal films selectively on predetermined portions by plating or the like. A plating resist film that covers the portions where the conductor wiring should not be formed is also formed generally by lithography.
As a result, although the semi-additive method is less expensive than the subtractive method due to the use of a simple film as the board, it still involves a large number of processes including the plating processes, thus resulting in low productivity of manufacturing the printed wiring board. In addition, use of the expensive resist still leads to high manufacturing cost.
Moreover, in case the conductor wiring formed by the semi-additive method includes narrow lines and wide lines, wider lines tend to become thicknesser and it becomes difficult to form the whole conductor wiring with uniform thickness.
In order to solve these problems and manufacture a printed wiring board with lower cost, such a manufacturing method has been in practice as an electrically conductive paste that contains fine metal particles and a binder made of resin or the like is directly applied to the board surface by screen printing or other printing method so as to form conductor wiring (for example, refer to Japanese Laid-Opened Patent Publication H06-68924-A (1994)).
However, while it is attempted to form the conductor wiring from fine lines measuring 100 μm both in the width and the space between the lines by the printing method using an electrically conductive paste in order to meet the demands for finer conductor wiring, the line width of the conductor wiring actually formed tends to be larger than the intended width.
As a result, such problems occur as adjacent lines become too close to or contact each other, or the lines have blurred edges, thus making it unable to form satisfactory conductor wiring.
There is also such a problem that, in an interface for connection with an external circuit provided in the conductor wiring for use as terminals to connect with component leads or connectors, or as contacts of film switches intended for a flexible printed wiring board, electrical resistance of the conductor wiring becomes higher and mechanical strength becomes lower as the lines width and space are made smaller.
In order to achieve lower resistance and increase the mechanical strength in the connecting portion, it has been proposed to form a metal plating film on the surface of conductor wiring that has been formed by printing (for example, refer to Japanese Laid-Opened Patent Publication S60-258631-A (1985)).
Recently the use of anisotropically conductive film and anisotropically conductive paste has been increasing for the connection of electrical and electronic components because these materials enable simple connection work with fine pitches. Such connection methods make it necessary to process the surface of the conductor wiring formed by printing method so as to make the joint suitable for the connection method, for example by applying gold plating.
Further, in order to improve the electrical conductivity of the entire conductor wiring formed with the electrically conductive paste, the entire surface thereof is coated with a plating layer (for example, refer to Japanese Laid-Opened Patent Publication H11-224978 (1999)).
However, it is difficult to satisfactorily form a continuous film by plating directly on the conductor wiring that is formed from the electrically conductive paste either on the portion thereof used for connection with an external circuit or over the entire surface thereof, without any treatment applied to the surface in advance.
Even when a plating layer is formed, it may not be bonded with the conductor wiring with sufficient strength.
When a flexible printed wiring board that comprises a board made of a flexible film and is subjected to bending during use experiences such problems as described above, peel off of layer and/or line breakage can occur when the printed wiring board is bent.
These problems become more serious when the line width of the printed wiring board is made smaller. This is because line width becomes smaller and the area of contact with the board or the plating layer becomes smaller, resulting in lower bonding strength, as the line width of the printed wiring board is made smaller.